Seaside Naturalist

Walk along any quiet cove and you are likely to find one: the hollowed, honeycombed carcass of what was once a proud plank or piling reaching out to the sea. Cracked and broken, these brittle remains can be found from Belfast to York and beyond, providing undeniable proof that a weird, homely little mollusk has staked its claim in our territory -- and continues to eat its way into our culture.

The mollusk is one Teredo navalis -- commonly known as the shipworm. With dozens of different species ranging across the globe, shipworms as a group have developed an intimate relationship with humans since we first put out to sea. In fact, these gregarious little creatures have been munching our hulls and pilings for centuries, their distinctive feeding tunnels having been found in buried Greek and Roman ships more than 2,000 years old.

With such a long history of destructive behavior, one might think that modern humans would have had time to overcome the threat of damage-by-clam that the shipworm represents. Given our love of technology and intolerant attitude toward pesky invertebrates, it would seem that some resourceful human should have found some solution that would prevent the pests from causing any further damage. After all, the wood-hungry "worms" barely have two brain cells to rub together.

The intensive maintenance that windjammers receive make them all but immune to shipworm attack, but battling Teredos has been a much tougher job than one might expect, especially when it comes to protecting pilings, floats, and other structures. Joining the ranks of the black fly, the shipworm has become one of those organisms that Mainers just can't seem to be rid of -- appearing just long enough to become truly irritating and then disappearing again. The last significant infestation of shipworms was centered in Northport in the 1950s, when hungry hordes chewed through native oak pilings with a seemingly insatiable appetite. After practically disappearing for almost 50 years, the nasty little creatures have reared their ugly head-like protrusions in recent years -- causing tens of thousands of dollars in damage.
 

A chunk of wood from a piling riddled with shipworm holes. (Photo by David Munson)

The main reason the shipworm has been so successful is a simple one: they are just plain good at what they do. As lowly and disgusting as they may seem, the shipworm is actually a highly-specialized, highly-evolved creature that fits into its niche nearly perfectly. A relative of clams and snails, the shipworm has a long, muscular foot but a greatly reduced shell, allowing it to work its body deep into the wood on which it feeds. Completely sessile as an adult, the mature shipworm lines its tunnel with a shell-like material and secures its place at the dinner table with a permanent ligament. With water-pumping siphons protruding from its tiny entrance hole, it spends its life just a tunnel wall away from dozens of other shipworms, devouring wood around the clock like a scene from Pinocchio's worst nightmare.

But, as any obsessive pencil-chewer knows, feeding on wood is easy; it's digesting wood that's hard. Like any other form of cellulose, wood can be a difficult material to break down, and the many animal species that feed on cellulose had to evolve fairly complex digestive systems to get the job done. Shipworms have solved the problem by getting friendly with a few key species of bacteria. In exchange for the comfort and protection of the shipworm's tissues, the bacteria break down the cellulose in the shipworm's meal.

Despite its fascinating biology, few people appreciate the intricacies of shipworm life. To most, the shipworm is a worm in the most classical sense -- a lowly, miserable creature worthy only of contempt. But, despite its critics, the shipworm has flourished, residing in one form or another in nearly every port.

One of the more successful species, the local Teredo navalis species can itself be found the world over, at least partly as a result of its unique reproductive strategy. Unlike many of its peers, T. navalis doesn't simply leave the future of its offspring to chance, but rather nurtures them internally until just a few short weeks before they are ready to commit themselves to a piece of wood. While most mollusk larvae are cut loose at their very conception to drift pell-mell through the ocean currents, T. navalis larvae are protected from predation inside the adult shipworm through their earliest developmental stages -- increasing their chances for survival. The destructive efficiency of the shipworm has inspired a number of interesting techniques for avoiding their hungry jaws. During the days of the big wooden sailing ships, it was not at all uncommon for a boat to be abandoned due to "worm rot" in the hull.

In fact, during his fourth trip to the New World, Christopher Columbus was forced to abandon two of his four ships due to irreparable shipworm damage. To avoid this, ships were often slathered in tar and covered with an additional layer of planking. This outer layer simply became a first course for the shipworms, serving only to delay the inevitable. Methods for applying copper sheathing were perfected in the 1700s, creating an expensive, but effective way to protect wooden ships from an untimely end.

With the popularization of metal and fiberglass hulls, fewer and fewer boats remain on the menu for shipworms. Outbreaks in Maine in recent years have led to only a couple of hull-breaches so far, both of which were due to a lack of protective paint on the vessel. The real battle is now being waged on the piers, pilings and floats along the coast. More effective pressure treatments, composite plastics, and naturally resistant tropical wood seem to be curbing the damage, and may even keep the shipworm army at bay -- for now.

But the little clams-gone-bad are nothing if not patient, and it's a good bet that they'll be back in the next decade, once we let our guard down.